[GIT PULL] uprobes: tmpfs support

[GIT PULL] uprobes: tmpfs support

[Oleg Nesterov]

Ingo, please pull from

  git://git.kernel.org/pub/scm/linux/kernel/git/oleg/misc uprobes/core

Based on tip:perf/uprobes


Usually I try to accumulate more changes before I ask to pull, but v3.15
is close and it would be nice to merge the tmpfs fix too. Plus a purely
cosmetic change I promised to do when we discussed the x86 changes.

Oleg Nesterov (3):
      uprobes: Shift ->readpage check from __copy_insn() to uprobe_register()
      uprobes: Teach copy_insn() to support tmpfs
      uprobes/x86: Rename arch_uprobe->def to ->dflt, minor comment updates

 arch/x86/include/asm/uprobes.h |    2 +-
 arch/x86/kernel/uprobes.c      |   37 ++++++++++++++++++-------------------
 kernel/events/uprobes.c        |   17 +++++++++++------
 3 files changed, 30 insertions(+), 26 deletions(-)

Re: [GIT PULL] uprobes: tmpfs support

[Christoph Hellwig]

On Mon, Jun 02, 2014 at 04:14:06PM +0200, Oleg Nesterov wrote:
> Ingo, please pull from
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/oleg/misc uprobes/core
> 
> Based on tip:perf/uprobes

Eww, adding tmpfs-specific code to uprobes screams layering violation.

Hugh, what is the problem with implementing ->readpage for tmpfs again?

Re: [GIT PULL] uprobes: tmpfs support

[Hugh Dickins]

On Mon, 2 Jun 2014, Christoph Hellwig wrote:
> On Mon, Jun 02, 2014 at 04:14:06PM +0200, Oleg Nesterov wrote:
> > Ingo, please pull from
> > 
> >   git://git.kernel.org/pub/scm/linux/kernel/git/oleg/misc uprobes/core
> > 
> > Based on tip:perf/uprobes
> 
> Eww, adding tmpfs-specific code to uprobes screams layering violation.
> 
> Hugh, what is the problem with implementing ->readpage for tmpfs again?

The problem is that ->readpage invites the caller to allocate a page
of their choice for pagecache, and then pass it down to the filesystem
to fill and use thereafter.

There are several ways in which that does not suit tmpfs, 3 spring to mind:
1. the page may already be in memory, but currently in swapcache not in
filecache: tmpfs knows how to manage that, the ->readpage caller does not
2. there may be a NUMA mempolicy applied to that file, which would choose
to allocate the page differently: tmpfs knows about that, caller does not
3. (handy side-effect) it happens to disable use of tmpfs file as swapfile

It was a great relief when tmpfs could finally jettison its ->readpage
back in v3.1 (though if you press me, I could admit to some remaining
embarrassments).  I certainly do not want it back.

Just think of tmpfs as a layering violation itself (memory as backing!
no wonder it has peculiar demands on the allocation of its backing)
and we're all good - there's a variety of ways in which the generic
code already happens to accommodate it (many PageSwapBacked tests,
or the mapping_cap_account_dirty/writeback tests, for example).

IIRC, you were in on the discussion of shmem_read_mapping_page() when we
introduced it: Oleg is simply adding a call to it to fix a uprobes bug.  
That the name explicitly mentions shmem instead of concealing it,
is not necessarily a bad thing.

Hugh

Re: [GIT PULL] uprobes: tmpfs support

[Oleg Nesterov]

On 06/02, Christoph Hellwig wrote:
>
> On Mon, Jun 02, 2014 at 04:14:06PM +0200, Oleg Nesterov wrote:
> > Ingo, please pull from
> >
> >   git://git.kernel.org/pub/scm/linux/kernel/git/oleg/misc uprobes/core
> >
> > Based on tip:perf/uprobes
>
> Eww, adding tmpfs-specific code to uprobes screams layering violation.
>
> Hugh, what is the problem with implementing ->readpage for tmpfs again?

I leave this to you and Hugh.

But I hope you are not arguing with this patch, it is very simple and we
do want to support tmpfs. If tmpfs has ->readpage again we can revert this
patch.



BTW. Is it safe to pass file == NULL to read_mapping_page() and ->readpage()?
Last time I tried to check this looked safe but this is not documented. We
need to call read_mapping_page() from uprobe_register() but it doesn't have
"struct file *" and thus we need to delay arch_uprobe_analyze_insn() until
we find the first mapping, too bad.

And why read_mapping_page() has "void *data" but not "struct file *file"...

Oleg.